1. Field of the Invention
The present invention relates to a method and apparatus for configuring Reference Signal (RS) to be transmitted by a relay through wireless backhaul in a Orthogonal Frequency Division Multiplexing (OFDM) system using multiple carriers.
2. Description of the Related Art
OFDM is a transmission technique for transmitting data using multiple carriers, i.e. multicarrier data transmission technique which parallelizes the serial input stream into parallel data streams and modulates the data streams onto the orthogonal multiple carriers, i.e. sub-carrier channels.
The origins of multicarrier modulation scheme started in the late 1950's with the microwave radio for military communication purpose, and OFDM using orthogonal overlapping multiple subcarriers has been developed in 1970's but limited in applying to the real systems due to the difficult of implementing orthogonal modulations between multiple carriers. With the introduction of the idea of using a Discrete Fourier Transform (DFT) for implementation of the generation and reception of OFDM signals, by Weinstein, in 1971, the OFDM technology has developed rapidly. Additionally, the introduction of a guard interval at the start of each symbol and use of cyclic prefix (CP) overcomes the negative effects caused by multipath signals and delay spread.
Owing to such technical advances, the OFDM technology is applied in various digital communications fields such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). That is, the implementation of OFDM could be accomplished by reducing implementation complexity with the introduction of various digital signal processing technologies such as Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT).
OFDM is similar to Frequency Division Multiplexing (FDM) but much more spectrally efficient for achieving high speed data transmission by overlapping multiple subcarriers orthogonally. Due to the spectral efficiency and robustness to the multipath fading, OFDM has been considered as a prominent solution for broadband data communication systems.
Other advantages of OFDM are to control the Inter-symbol Interference (ISI) using the guard interval and reduce the complexity of equalizer in view of hardware as well as spectral efficiency and robustness to the frequency selective fading and multipath fading. OFDM is also robust to the impulse noise so as to be employed in various communication systems.
In wireless communications, high-speed, high-quality data services are generally hindered by the channel environments. In wireless communications, the channel environments suffer from frequent changes not only due to additive white Gaussian noise (AWGN) but also power variation of received signals, caused by a fading phenomenon, shadowing, a Doppler effect brought by movement of a terminal and a frequent change in a velocity of the terminal, interference by other users or multipath signals, etc. Therefore, in order to support high-speed, high-quality data services in wireless communication, there is a need to efficiently overcome the above channel quality degradation factors.
In OFDM, modulation signals are located in the two-dimensional time-frequency resources. Resources on the time domain are divided into different OFDM symbols, and are orthogonal with each other. Resources on the frequency domain are divided into different tones, and are also orthogonal with each other. That is, the OFDM scheme defines one minimum unit resource by designating a particular OFDM symbol on the time domain and a particular tone on the frequency domain, and the unit resource is called a Resource Element (RE). Since different REs are orthogonal with each other, signals transmitted on different REs can be received without causing interference to each other.
Physical channel is a channel defined on the physical layer for transmitting modulation symbols obtained by modulating one or more coded bit sequences. In an Orthogonal Frequency Division Multiple Access (OFDMA) system, a plurality of physical channel can be transmitted depending on the usage of the information sequence or receiver. The transmitter and receiver determine REs on which a physical channel is transmitted, and this process is called mapping.